Currently, the scanner has been widely used for scanning and outputting images. Since the optical properties of the image-capturing device in the scanner are degraded over time, a calibration or compensation operation for adjusting the gains and offsets for the respective pixels of the scanner should be executed. Consequently, the output images having the same quality as the original scanned objects are obtained. Namely, the quality of scanned images is directly related to the quality of the calibration. Typically, whether in handheld scanners, flatbed scanners or automatic sheet-fed scanners, the calibration is accomplished by utilizing the image capturing device to scan an area of a calibration piece with known optically properties dynamically, so as to obtain a compensation value for the successive image processing operation accordingly.
For example, when a handheld scanner is used, the user can use a calibration piece attached to perform calibration. The user holds the handheld scanner and moves the image capturing device of the scanner on the calibration piece so as to capture the image information of the calibration piece. Consequently, pixel compensation values for the successive image processing operation are obtained. In order to facilitate the user to accomplish the calibration, the size of the calibration piece for the handheld scanner is larger than that of the flatbed scanner. Consequently, the calibration piece for the handheld scanner is not easy to be preserved and carried. In addition, while performing the calibration operation, the user has to hold the handheld scanner to scan. The handheld scanner can't execute the calibration operation automatically, so that it can't be utilized conveniently. Moreover, contamination for example dust or dirt covers the calibration piece easily, and creases may be formed on the calibration piece. Hence, when the handheld scanner scans the above-mentioned calibration piece (i.e. dust, dirt or creases on the calibration piece occurs), the compensation values calculated for the respective pixels of the scanner during the calibration will be incorrect. Consequently, the accuracy of the successive image calibration and the quality of the outputting image are adversely influenced.